The invention relates to a method and an arrangement for tuning a combiner filter.
One known way to connect several radio transmitters to the same antenna or antenna line is to connect each radio transmitter through a separate bandpass filter having its centre frequency tuned to the transmitting frequency of the radio transmitter. Such bandpass filters are called combiner filters. The function of each combiner filter is to feed the transmitting signal of each respective radio transmitter to the antenna with the smallest possible losses and prevent the leakage of transmitting signals of different frequencies from other radio transmitters, from the direction of the antenna, to this particular radio transmitter as efficiently as possible. Conventionally, combiner bandpass filters are tuned fixedly to the transmitting frequencies of the radio transmitters. So it has not been possible to change the transmitting frequency of the radio transmitter without simultaneously changing the combiner filter or its tuning.
In certain cases, however, it is desirable that the frequencies of radio transmitters can be changed simply and rapidly. One such case is the base station of a cellular mobile radio system to which predetermined transmitting and receiving channels have been assigned. In cases where the channel allocation of the system can be altered, if desired, by varying the transmitting and receiving frequencies of the base stations, the channel capacity of the system can be utilized in a flexible and efficient way in varying conditions. Therefore combiner filters have been developed in which the centre frequency changes automatically with the transmitting frequency.
The adjustment of known automatically tunable combiner filters is based on measuring RF power reflected from the input of the filter or RF power passing through the combiner filter, and on locking to a minimum/maximum value of the measured power. A problem with this adjustment method is, however, the low adjusting accuracy and the narrow dynamic range. As the frequency selectivity of the entire adjusting system relies on the combiner filter, power components of other radio transmitters leaking through the combiner filter to its input cause the minimum reflection suppression value of the reflection suppression measurement at the filter input to be about 7 dB, which results in a narrow dynamic range for the measurement. In the adjustment method based on the measurement of the power passing through the combiner filter, the measuring dynamics of the maximum power value also remain low for the same reasons as above. In addition, the automatically tunable combiner filter based on this prior art adjustment method does not allow variation in the relative power levels between the radio transmitters, that is, the "mutual dynamics" are almost 0 dB, as a change in the power level of one transmitter affects immediately the power measurement in the adjustment circuits of the combiner filters of the other radio transmitters, thus causing adjustment error.
Finnish Patent Application 912 255 discloses an adjusting arrangement where the frequency selectivity of the measurement of the RF power passing through or reflected has been increased by mixing the RF measuring signal with a signal of the same frequency, i.e. a transmitting frequency signal, so that a substantially zero-frequency signal, i.e. a DC signal, is obtained, which has a level proportional to the RF power reflected from the input or to the RF power passing through the filter. Power components leaking from the transmitting frequencies of other radio transmitters appear in the mixing result, e.g. at a frequency of 300 kHz or at higher frequencies, which are filtered off in the lowpass filtration following the mixing. In this way the invention provides a measuring signal the level of which depends merely on the transmitting-frequency RF power component to be measured, whereas the measuring signal is not affected by the transmitting signals of the other radio transmitters.